[PATCH] fs/exec: require argv[0] presence in do_execveat_common()

[Ariadne Conill]

The first argument to argv when used with execv family of calls is
required to be the name of the program being executed, per POSIX.

By validating this in do_execveat_common(), we can prevent execution
of shellcode which invokes execv(2) family syscalls with argc < 1,
a scenario which is disallowed by POSIX, thus providing a mitigation
against CVE-2021-4034 and similar bugs in the future.

The use of -EFAULT for this case is similar to other systems, such
as FreeBSD and OpenBSD.

Interestingly, Michael Kerrisk opened an issue about this in 2008,
but there was no consensus to support fixing this issue then.
Hopefully now that CVE-2021-4034 shows practical exploitative use
of this bug in a shellcode, we can reconsider.

Signed-off-by: Ariadne Conill <ariadne@xxxxxxxxxxxxxxxx>
---
 fs/exec.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/fs/exec.c b/fs/exec.c
index 79f2c9483302..de0b832473ed 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1897,8 +1897,10 @@ static int do_execveat_common(int fd, struct filename *filename,
 	}
 
 	retval = count(argv, MAX_ARG_STRINGS);
-	if (retval < 0)
+	if (retval < 1) {
+		retval = -EFAULT;
 		goto out_free;
+	}
 	bprm->argc = retval;
 
 	retval = count(envp, MAX_ARG_STRINGS);
-- 
2.34.1